The present invention relates to a method of stabilizing pressure and flow conditions in a screening apparatus for treating a pulp of fibrous cellulosic material. The invention also relates to a screening apparatus for performing this method.
U.S. Pat. No. 4,396,509 describes a screening apparatus of pressure diffuser type with a screen moving in a cycle to and fro and being journalled on two cylindrical support bodies having different diameters at the inlet and outlet sides of the displacement zone. At the return movement of each cycle the screen executes a rapid return stroke, the volume of the filtrate or extraction space being reduced as a result of said differences in the diameters of the support bodies. The screen is thus backflushed with a liquid volume corresponding to the difference in cross-sectional areas of the two cylindrical support bodies multiplied by the stroke length of the screen minus the quantity of displaced liquid removed from the screening apparatus during the return stroke. During the rest of the cycle, i.e. the extraction phase, the screen moves in the same direction, and at substantially the same speed, as the pulp. In this known method the pulp is moved at the correct speed in relation to the screen at the same time as clogging of the screen holes is prevented. However, the distribution of backflushing is not satisfactory. One solution to this problem is to design the screen as a truncated cone, as described in U.S. Pat. No. 4,368,628.
In the known screening apparatus described in the above-mentioned patents, the pulp flows continuously into and out of the displacement zone at the same time as displacement liquid is supplied radially from the outside in the direction to the screen surface and extracted or displaced liquid is allowed continuously to leave the apparatus. As a result of the in and out flow of the pulp the pressure in the pulp will increase at the inlet end of the screen or displacement zone and drop at its outlet end during the return stroke of the screen.
A pressure gradient thus exists between the inlet end and outlet end of the displacement zone. This circumstance may prevent the backflushing from being correctly distributed since no corresponding pressure gradient occurs in the space for the displaced liquid. Most of the backflushing will therefore occur close to the outlet end of the displacement zone, whereas little or no backflushing will occur in the direction to the inlet end of the screen.